1. Field of the Invention
The present invention relates to a light source apparatus and an inspection apparatus.
2. Description of Related Art
In the manufacturing process of a photomask for semiconductor exposure, for which a fine processing is required, it is necessary to inspect microscopic defects that may be contained in the photomask. Further, in the case of a reticle on which a fine exposure pattern is written, an inspection must be performed to check as to whether the actual pattern is written correctly and whether there are any defects in the pattern.
For such purposes, a semiconductor defect inspection apparatus that illuminates the object under inspection with continuous or high repetition pulsed light and detects a change in a light intensity due to scattering or the like for comparison is used. Such semiconductor defect inspection apparatus is available in a variety of types, but regardless of the type, the resolution generally increases as the wavelength of the light source is made shorter. Accordingly, the light source apparatus of the type that produces inspection light in the deep ultraviolet region of the spectrum by wavelength conversion of laser beams using nonlinear optical crystals has come into wide use in recent years. As a principle of the apparatus that detects a change in a light intensity, a light source output needs to be always stable in both the short term and the long term, and thus there have been various methods to control the light source output so as to stabilize it.
Japanese Patent No. 4654424 discloses a light source apparatus that generates light at 213 nm or 266 nm using nonlinear optical crystals. The light source apparatus disclosed in Japanese Patent No. 4654424 uses an element, as means for stabilizing an ultraviolet light output, that changes a refractive index of a fundamental light beam by a photoelastic effect or an electro-optical effect. To be more specific, polarizing adjusting means using the electro-optical effect is disposed in an optical path of an incident light beam entering a nonlinear optical crystal.
Japanese Unexamined Patent Application Publication No. 2005-25054 discloses an apparatus that measures a portion of laser beam and provides feedback control on an acousto-optical element (AO element). More specifically, a first-order diffracted light beam of the AO element is used as an output light beam.
In the technique disclosed in Japanese Patent No. 4654424, as a voltage or pressure is continuously applied to an electro-optical element, which is the polarization adjusting means, the polarization adjusting means could be easily degraded and destroyed, and a performance thereof could be readily reduced. Additionally, as the element itself is expensive, frequent exchange of it will lead to an increase in the cost.
Japanese Unexamined Patent Application Publication No. 2005-25054 discloses a method for controlling and stabilizing an output by the AO element. However, when a zero-order light beam with a small loss is used, a beam profile is degraded due to diffraction, and a quality of a laser beam changes, thereby degrading the performance. For this reason, a first-order diffracted light beam is used as an output light beam. In most cases, as an efficiency of the first-order diffracted light beam is limited to about 0 to 80% of an amount of an incident light beam, a maximum output is reduced by 20% with respect to an input light beam. When a wavelength of such a light beam is converted, a conversion efficiency is further reduced. Further, as the first-order diffracted light beam is output with an angle (twice as great as the Bragg angle) with respect to the incident light beam, it is inconvenient to implement this method. Also, with a short wavelength, materials of the AO element are limited, thereby greatly reducing a diffraction efficiency.
The present invention has been made in light of such circumstances, and an object of the present invention is to provide a light source apparatus having a simple configuration that can stably generate wavelength converted light beams without greatly reducing a generated output and an inspection apparatus having a simple configuration.